In 1979, the IEEE published an article which has become a basic reference in the field of television echo (or "ghost") elimination. The article is entitled "A Tutorial On Ghost Cancellation In Television Systems" and was written by Walter Ciciora, Gary Sgrignoli and William Thomas and it is incorporated by reference herein.
Although the Ciciora article described the fundamental principles, apparatus and algorithms applicable to ghost cancellation, the state of the art has only recently progressed to the point of providing practical ways to implement and improve these basic concepts.
There are two main steps to the ghost cancellation process. First the characteristics of the communications path or channel (which include the echo artifacts, if any) must be determined at the receiver. Once the channel characteristics are calculated, filters are used to implement the inverse channel characteristics to substantially eliminate the ghosts.
A straight forward approach to determining the characteristics of a transmission path is to transmit a ghost cancellation reference signal (GCR) over the path. The originally transmitted GCR signal is compared with the received, and "ghosted", GCR signal, and a model of the characteristics of the communications path is developed based on the comparison.
U.S. Pat. No. 5,121,211 describes an improved ghost cancellation reference signal and a system utilizing such a signal for cancelling ghosts. The GCR signal described in the '211 patent is transmitted on a single line of the vertical blanking interval of the television signal. One feature of this GCR signal is that it has a higher energy level with peaks more evenly distributed in time than other GCR signals which have been proposed such as the BTA GCR signal. The '211 GCR thus provides the flat, wide frequency spectrum necessary to fully characterize the path and has a high energy level (amplitude) and a more even distribution of energy over a time interval.
Parent application '927, describes an improved ghost cancellation circuit which comprises FIR and IIR filters which operate at a higher speed by reducing the time in which it takes to generate the appropriate filter coefficients. As discussed in that application, proposals have been made to transmit a GCR signal in one line of each field in an eight field sequence. The received GCR signals from the eight fields are averaged at the receiver and the average value of the received GCR is then compared with the stored reference GCR in the receiver's ghost cancellation circuit in order to derive the necessary filter coefficients for characterizing the channel. By using a value avereaged over a given number of fields, variations which take place within each individual field are minimized and a more accurate representation of the channel can be provided.
The path characterization process can be hindered due to the presence of sync and color burst signals present on a television line in close proximity to the GCR signal. Ghosted versions of these signals can overlap the portion of the received television line containing the GCR. DC level shifts can cause further distortion to the received GCR signal. The BTA of Japan has developed a method which seeks to eliminate the effects of these signals and their ghosts on the received GCR signal. U.S. Pat. No. 5,130,799 describes a system and method for ghost cancellation using this type of eight field sequence.
Since the color burst signal changes phase by 180.degree. from frame to frame, the color burst signal will be at the same phase in lines four fields, or two frames, apart. The BTA method suggests using an eight field sequence wherein four fields are transmitted with a television line comprising a GCR signal, and four fields are transmitted with a television line without the GCR signal. At the receiver, lines of the sequence received four fields apart, and having color burst signals of opposite phase, are subtracted from each other. This combination yields a GCR signal value averaged over the eight fields, unaffected for the most part by the ghosts caused by the color burst signal. In addition, this subtraction eliminates the DC drift, sync signals and other types of pair-wise constant reference or test signals present on one or more lines adjacent to the line chosen to provide the GCR signal. The term "pair-wise constant" will be used herein to refer to signals which are constant on lines a fixed number of fields apart, for example four fields apart.
In accordance with the BTA sequence, the GCR signal is actually transmitted only in four of the eight television lines of the sequence and thus the average GCR signal value which is used to characterize the path at the receiver is limited somewhat in robustness. In addition, the sequence and apparatus described by the '799 patent is not useful in cancelling the effects of test signals which are not pair-wise constant, in otherwords signals which change in between fields, or signals which do not appear in pairs of lines within the sequence, for example teletext and SID signals.
It is therefore an object of the instant invention to provide a method and apparatus for minimizing or eliminating the effects of pair-wise constant television line signals as well as those non-pair-wise line signals which may appear in within each sequence of fields.
Another object of the invention is to provide a more robust GCR signal thus improving the signal to noise ratio of the ghost cancellation system.